Generally, a surface acoustic wave device comprises a piezoelectric substrate, together with an input electrode and an output electrode, which comprise comb-tooth electrode fingers disposed on the piezoelectric substrate. When an electric signal is input to the input electrode of the surface acoustic wave device, an electric field is generated between the electrode fingers, and surface acoustic waves are excited due to the piezoelectric effect and propagated over the piezoelectric substrate. There has been under research a surface acoustic wave sensor for detecting various substances and measuring properties thereof, which incorporates therein a surface acoustic wave device that utilizes shear horizontal surface acoustic waves (SH-SAW) among the excited surface acoustic waves referred to above. Shear horizontal surface acoustic waves are displaced in a direction perpendicular to the direction of propagation of surface acoustic waves (Japanese Patent No. 3481298).
Operation of the surface acoustic wave sensor utilizes the fact that output signals output from the output electrode have different characteristics when a region of the piezoelectric substrate where a liquid material to be measured is placed is electrically open compared with when the same region of the piezoelectric substrate is electrically short-circuited. More specifically, the output signal produced when the region of the piezoelectric substrate is open is subjected to both electrical and mechanical interactions, whereas the output signal produced when the region of the piezoelectric substrate is short-circuited is subjected only to mechanical interactions. Therefore, physical properties of the liquid material, such as a dielectric constant, electric conductivity, etc., can be determined by canceling out mechanical interactions, and then extracting electrical interactions from the output signals.
For measuring physical properties of a liquid material with the surface acoustic wave sensor, there are instances where the surface acoustic wave sensor is immersed in the liquid material. In such applications, the electrodes of the surface acoustic wave device of the surface acoustic wave sensor are sealed by sealing members in order to prevent the electrodes from becoming short-circuited by the liquid material.
If the sealing members are disposed on the propagation path of the surface acoustic waves, then since the thickness of the sealing members in the direction of propagation is determined by propagation characteristics of the surface acoustic waves, the sealing member must be formed with great accuracy. Furthermore, the sealing members that seal the electrodes may become peeled off from the piezoelectric substrate upon elapse of a certain period of time, with the result that the liquid material may find its way into gaps between the peeled-off sealing members and the piezoelectric substrate, thus tending to short-circuit the electrodes.